In high-frequency communication systems typified by mobile phones, used are high-frequency filters to remove unnecessary signals in frequency bands other than the frequency band used for communication. As the high-frequency filter, used is an acoustic wave resonator such as a surface acoustic wave (SAW) resonator. In the SAW resonator, a metal grating electrode is formed on a piezoelectric substrate such as a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate.
The grating electrode excites a Shear Horizontal (SH) wave, a Rayleigh wave, or a boundary acoustic wave, which is a type of surface acoustic waves. Reflectors located at both sides in a direction in which acoustic waves excited by the grating electrode mainly propagate confines the acoustic waves to the grating electrode. Ladder-type filters and multi-mode filters can be formed with use of acoustic wave resonators. There has been known an acoustic wave resonator that weights a width of the grating electrode in a direction perpendicular to the propagation direction of the acoustic wave as disclosed in Japanese Patent Application Publication Nos. 9-270667 and 2008-78883.
In the acoustic wave resonator including the grating electrode, a lateral-mode spurious, which is unnecessary response, occurs. The lateral-mode spurious occurs when acoustic waves containing a component in a direction perpendicular to the propagation direction of the acoustic wave intensify each other at a certain wavelength. In Japanese Patent Application Publication Nos. 9-270667 and 2008-78883, the cross width differs depending on the position in the propagation direction of the acoustic wave, and thus the frequency at which the lateral-mode spurious occurs differs depending on the position in the propagation direction. Thus, the frequencies at which the acoustic waves in the lateral mode intensify each other are averaged, and the lateral-mode spurious is reduced. However, this does not mean that the generation of the acoustic wave in the lateral mode is inhibited. Thus, the acoustic wave in the lateral mode leaks to the outside of the grating electrode. Therefore, loss can be caused.